This invention relates to body implantable leads and, in particular, pacing leads having a distal fixation element for attachment to the inner wall of a patient's heart.
The efficacy of cardiac pacing has been widely accepted for some time now. As is well known, a pacing system comprises the basic combination of a generator, or pacemaker, and a lead. The pacemaker performs the basic function of generating pacing pulses, and also receives sensed heartbeat signals and other sensor signals for determining when pacing pulses are to be delivered and at what rate. The pacemaker is operatively connected to the heart tissue by the lead, which carries the generated pacing pulses to the heart and carries sensed heart signals from the heart back to the pacemaker. The pacing lead is fixed to the pacemaker at the proximal end by known techniques, and the distal end must be positioned at a desired location adjacent to heart wall in order to secure optimized chronic performance. In order to secure the distal end of the lead to the heart wall, many leads employ an anchoring element such as a helical coil, barbs or the like. A great many different anchoring elements are known in the art and are disclosed in the patent literature. For the purposes of this invention, the helix, or screw-in element, will be used as illustrative, it being understood that the invention is not limited and is applicable to other types of anchoring elements.
In practice, the physician must introduce the lead intravenously into the heart, position the distal tip adjacent the heart wall so that an optimum threshold is obtained and then, in the case of a lead with an active anchor element, secure the element into the heart wall. The intravenous introduction of the lead requires that the lead be flexible and small in diameter, and devoid of any protruding element or part which would hinder passage through the vein. Thus, the presence of a distal anchor element can carry the substantial disadvantage of making the intravenous insertion much more difficult. In order to overcome the disadvantage posed by the anchor element to the insertion process, a number of lead designs have been proposed and implemented, with varying degrees of success. Recently, there has been disclosed a cardiac pacing electrode having a soluble covering which surrounds the fixation helix which is mounted at the distal tip end. Such a covering has size, shape and solubility characteristics such that it maintains its smooth outer form during the transvenous insertion process, but thereafter dissolves so as to expose the anchor helix or other like anchor element. Reference is made to U.S. Pat. Nos. 4,827,940 and 4,876,109, which disclose such a soluble covering for a cardiac pacing electrode. The problem that remains with the covering as described in the aforementioned two patents, is that the implantation procedure is necessarily interrupted while the physician waits a sufficient time to permit full dissolution of the soluble coveting. In practice, the physician, or physician's assistant, has to either wait a specified time or make measurements such as impedance drop, before screwing the lead into the heart tissue. For an experienced and adept implanter, this introduces an annoying delay, since invariably it is necessary to overwait in order to ensure that the soluble covering has indeed dissolved. What is thus desired in the art is a lead with means for providing an instant indication as to when the covering has dissolved, so that the fixation element can be fixed to the heart wall without delay.
One approach to providing an indication as to when the covering has dissolved is disclosed in U.S. Pat. No. 5,531,783 issued to Giele et al., in which a mannitol covering is improved by having a contrast medium or like radiopaque material within at least a portion thereof to render it x-ray visible until it dissolves, whereby the lead tip can be monitored to determine first when the fixation element is proximate to the heart tissue, and particularly when the covering has dissolved. The process employed to fabricate the covering unfortunately requires a high temperature melting step and exposure of the lead to the mannitol covering while melted in order to apply the covering to the fixation element. Such high temperatures are particularly undesirable in leads which include a steroid drug for elution adjacent the fixation element in order to reduce pacing thresholds. Sodium dexamethasone phosphate, for example, is damaged by temperatures in excess of 140 degrees Fahrenheit. High temperatures may also cause decomposition of contrast media.